To synthesise non-oxide glasses for optical fibers for telecommunications, two types of furnaces are usually used: resistor furnaces and induction furnaces.
Induction furnaces present the advantage of guaranteeing a high heating speed and of favoring the good mixing of the components of the glass, thanks to the agitation induced on the ions present in the molten glass by the magnetic field used for heating. However, the structure of the furnaces is such as to give rise to relatively high temperature inhomogeneity in the mixture (differences in the order of 20-30.degree. C. between the coolest and the hottest point) and this does not allow obtaining a high quality of the glass and, consequently, of the resulting fiber. Moreover, since a lower temperature zone is found just in correspondence with the mouth of the crucible containing the mixture of the glass constituents, in case of mixtures whose constituents easily tend to sublimate (such as, in particular, mixtures for fluorozirconate glasses), sublimated materials are likely to become deposited around this mouth and those materials, when the molten mixture is cast, can introduce impurities in the glass.
Resistor furnaces, on the contrary, homogeneously heat the mixture, but the required temperature is reached more slowly and they do not favor good component mixing. Moreover, it is relatively complex to obtain heating in controlled atmosphere.